An infrared (IR) seeker is a device within a missile which homes on the IR signature of a target vessel (e.g. a ship, aircraft, etc.), decides whether it has detected a target, and then maintains a lock on the target. All matter emits IR radiation. The IR signature of a target and its surroundings is comprised of an IR signal that is spatially distributed.
The seeker may be part of an anti-ship missile launched outside the visible range of a ship (over horizon) with knowledge of ship's location, from radar or satellite for example. The missile may use Global Positioning System (GPS) guidance for most of its path, and then in the final stage, switch over to radar or IR detection.
Ships can be designed to have stealth in both the radar and IR bands.
An IR seeker can use as its signal: an integration of the IR signal from its field of view into a single number (e.g. if it uses just a single detector); or an image of its field of view (e.g. if it uses a focal plane array).
The seeker attempts to discern the target from the background or other objects in the field of view. This will only be possible once the target is visible, meaning there is a line of sight from the ship to the seeker allowing for altitude and earth curvature. For the purpose of this description, visibility does not equate with detectability. Detection will occur at a time that depends on how strong the signal from the ship is in the atmosphere, and that also depends on noise in the surroundings (sea surface reflections etc.). A seeker which discerns a target with sufficient certainty (e.g. using a signal to noise ratio metric) has “detected” the target.
An example of a simple seeker is one which aims towards the position in space which represents the strongest signal.
A ship may employ IR seeker countermeasures. An example of a countermeasure is a flare (incendiary) which is launched from the ship into the air and presents another possible target to the seeker. A successful flare is one which the seeker chooses to track instead of the ship.
A more sophisticated seeker is one which attempts to identify a space within the field view which encompasses the target. This space is called a “track gate”. The seeker can estimate the track gate, and update the track gate continuously using a two dimensional focal array. The seeker may store information about the track gate to increase the probability that the target stays within the track gate as the seeker advances towards the target.
The seeker continues to look for the target, and adapts the track gate to accommodate for the increase in effective size and signal intensity of the target as the distance to the target decreases.
An IR seeker can store information about the intensity of the IR signal within its track gate. This signal intensity changes with time as the distance to the target decreases. Keeping signal intensity in memory can be used by a seeker to reject (i.e. not include within its track gate) signals within its field of view which may not be part of the target.
For example, a seeker which maintains a track gate can reject a flare if its signal intensity is not close enough to what is recorded for the track gate.
If a seeker detects the target, it is likely to reject a flare by use of a track gate. However, if a seeker detects the flare, it is likely to reject the target. That is, the countermeasure is successful. Sometimes the countermeasure will be successful and others not.
It follows that a countermeasure is more likely to be successful if a flare can be launched prior to the seeker detecting the ship, so that the seeker is more likely to detect the flare. In some scenarios (e.g. lower resolution seeker focal plane array, or low observable environment) the distance at which a seeker detects a ship decreases with the decreasing ship IR signature. In these scenarios a low ship IR signature increases the probability that the flare can be launched prior to the ship being detected, and so a low IR signature increases countermeasure effectiveness.
A missile engagement situation in which a flare countermeasure is launched and is detected by the seeker prior to its detecting the ship is called a “distraction” scenario. An engagement situation in which a flare countermeasure is launched after the seeker has detected the ship is called a “seduction” scenario.
The IR signature of a ship is dependent on the environmental conditions and the ships operational state (which engines are running and at what power). In some conditions, the IR signature is low enough that it is likely that the flare can be launched prior to the ship being detected. However, in other conditions it is likely that the ship will be detected prior to the flare being launched. So for a given ship whether an engagement is a distraction or seduction scenario changes with the environmental conditions and the ship's operational state.
Conventional IR suppression systems are configured to reduce IR signature as much as possible either at all times or at times when the ship is under threat.
The conventional IR suppression systems increase the probability versus an unsuppressed ship that the ship will be in a distraction scenario versus a seduction scenario.
The conventional IR suppression systems may also increase the probability that the countermeasure is successful in a seduction scenario, but their effectiveness is limited due to the sophistication possible in the seeker hardware and algorithm, for example, through high resolution focal plane arrays and the use of track gates as described above.